1. Field of the Invention
The present invention relates to an optical amplifier used in a WDM (wavelength division multiplex) transmission system.
2. Description of the Related Art
Recently, along with the development of Internet technology, the amount of information to be transmitted through networks has been rapidly increasing. Therefore, particularly in trunk-line optical transmission systems, both a larger capacity and flexible network formation of transmission lines are required. As one of the most effective methods meeting these requirements, a WDM (wavelength division multiplex) transmission system has been commercialized. The wavelength-division multiplex transmission system is a technology to simultaneously transmitting a plurality of signals through an optical fiber and it can be implemented by multiplexing signal light with a plurality of different wavelengths. In North America and the like, the commercialization of the wavelength-division multiplex transmission system has already been promoted.
For an optical amplifier used in the wavelength-division multiplex transmission system, at present, a rare earth-doped fiber optical amplifier is the most popular in use. The rare earth-doped fiber optical amplifier provides optical amplification by adding a rare earth element to an optical fiber. For example, an erbium-doped fiber optical amplifier (EDFA) is widely known. Since the rare earth-doped fiber optical amplifier has a wide gain band and can collectively amplify multi-wavelength light, the amplifier is widely used as a key component for implementing the wavelength-division multiplex transmission system.
Although conventionally the erbium-doped fiber optical amplifier amplified mainly a band of 1530-1565 nm called a “C-band (conventional band)”, recently one for amplifying a band of 1570-1610 nm called an “L-band (long wavelength band)” is being developed. At present, a wavelength-division multiplex transmission system using an erbium-doped fiber optical amplifier can multiplex approximately 200 waves (channels) by using both C-band and L-band together.
In the design of an erbium-doped fiber optical amplifier, an inversion population ratio (or inversion population density) must be appropriately chosen so that the gain of each piece of signal light multiplexed in a wavelength band may be constant. The design method of an erbium-doped fiber optical amplifier is briefly described below.
FIG. 1 shows the wavelength dependence of the gain coefficient of an erbium-doped fiber optical amplifier, where the inversion population ratio of erbium is used as a parameter. The inversion population ratio is the ratio of the number of erbium elements transiting to an excitation state, to the total number of erbium elements added to an optical fiber. The gain coefficient is a gain obtained by a unit length of erbium-doped fiber. Therefore, input light is amplified in an area where a gain coefficient is positive, while the power of input light degrades in an area where the coefficient is negative.
As shown in FIG. 1, the gain of an erbium-doped fiber depends not only on the wavelength, but also on the inversion population ratio. Specifically, the larger the inversion population ratio, the larger the gain. The smaller the inversion population ratio, the smaller the gain. The following is known from the characteristic shown in FIG. 1.    (1) Since in the case of amplifying the C-band collectively, the gain-wavelength characteristic of the signal band must be flat, it is preferable for the inversion population of an erbium-doped fiber to be approximately “0.7”. If the inversion population ratio is “0.7”, a fairly large gain coefficient can be obtained. Therefore, the erbium-doped fiber optical amplifier for amplifying the C-band can secure a sufficient gain by using a fairly short optical fiber.    (2) Since in the case of amplifying the L-band collectively, the gain-wavelength characteristic of the signal band must be flat, it is preferable for the inversion population of an erbium-doped fiber to be approximately “0.4”. If the inversion population ratio is “0.4”, the gain coefficient is fairly small. Therefore, in order to make the gain of the L-band equivalent to the gain of the C-band, the fiber length of an erbium-doped fiber optical amplifier for amplifying the L-band must be longer to some degree.
FIG. 2 shows the basic configuration of an erbium-doped fiber optical amplifier for amplifying the L-band. This optical amplifier is often called a gain shift type erbium-doped fiber optical amplifier. The amplifier comprises an erbium-doped fiber 1 as an optical amplification medium, optical isolators 2-1 and 2-2, a wavelength-division multiplexer (ex. WDM coupler) 3 and a pump light source 4. Multi-wavelength light inputted from a transmission line is inputted to the erbium-doped fiber 1 through the optical isolator 2-1 and the WDM coupler 3. Here, pump light generated by the pump light source 4 is supplied to the erbium-doped fiber 1. Therefore, the multi-wavelength light is amplified by the erbium-doped fiber 1. Then, the amplified multi-wavelength light is outputted through the optical isolator 2-2. The configuration of an erbium-doped fiber optical amplifier for amplifying the C-band is basically the same as this amplifier for amplifying the L-band. However, the lengths of the two optical fibers as optical amplification media are different.
In this optical amplifier, the output power of the pump light source 4 is, for example, controlled by a feedback system for maintaining the output power of multi-wavelength light constant. Specifically, a part of multi-wavelength light outputted from the erbium-doped fiber 1 is guided to a control circuit 12 by an optical splitter 11. Then, this control circuit 12 controls the pump light source 4 in such a way that received multi-wavelength light can be maintained at a specific level.
In a wavelength-division multiplex transmission system, a communications channel can be set for each wavelength. Therefore, the configuration of a transmission system can be flexibly modified without installing a new optical fiber or changing the connection between optical fibers. To establish a flexible transport network, an optical communications system for add/drop an optical signal with a specific wavelength in a plurality of multiplex optical signals with a plurality of different wavelengths must be implemented.
However, when one of signal light among a plurality of signal lights in the L-band is turned off, the gain of the erbium-doped fiber optical amplifier for remaining signals varies. Specifically, if signal light on the short wavelength area in the L-band is turned off, the optical power of signal light on the long wavelength area in the L-band outputted from the erbium-doped fiber optical amplifier is lowered compared with that obtained when the signal light on the short wavelength area described above is inputted. In this case, the optical power of the remaining signal light on the long-wavelength area is lowered 10 dB or more, depending on conditions. Therefore, if this phenomenon occurs, there is a possibility that a receiver cannot receive the remaining signal light on the long wavelength area, which is a problem.
It is considered that this phenomenon is due to the fact that signal light on the short wavelength area in the L-band works as the pump light for signal light on the long wavelength area. In the following description, this phenomenon is called “output power changing phenomenon” or “deviation”.
The deviation can be theoretically solved by the feedback system shown in FIG. 2. Specifically, when the output optical power of an erbium-doped fiber 1 is lowered, increasing the power of the pump light source 4 by the control circuit 12 compensates for the deviation. However, in order to compensate for the deviation, a feedback system with a response speed in units of micro-seconds must be prepared, the implementation of which is difficult. Even if such a feedback system can be implemented, the stability of a control system would be a problem. Since this method aims to compensate for the deviation caused in an optical amplification medium by pump light feedback control, this cannot be a fundamental solution from the viewpoint of the optical static characteristic of an erbium-doped fiber optical amplifier. Specifically, this method cannot avoid deviation itself.
In order to avoid the deviation, there is, for example, a configuration to shorten the fiber length of an erbium-doped fiber optical amplifier. However, if the fiber length is shortened, as described with reference to FIG. 1, a sufficient gain cannot be obtained. In order to obtain a sufficient gain with a short fiber, higher inversion population ratio is required. However, a gain varies depending on a wavelength in a case where the inversion population ratio is changed, each signal multiplexed in the multi-wavelength light cannot be amplified by a same gain.
As another solution, there is a configuration in which a plurality of short erbium-doped fibers are connected in series. However, since in this configuration, the pump light source must be prepared for each erbium-doped fiber, it is difficult to implement the miniaturization and low cost of an optical amplifier.
This problem can occur not only in an erbium-doped fiber optical amplifier, but also in optical amplifiers in other forms.